"We have received information concerning an Internet security vulnerability named the Heartbleed Bug," the CRA, the government's tax collector, said. It added that the shutdown was a preventive measure.

Experts say the Heartbleed bug, found in widely used Web encryption technology, is one of the most serious security flaws uncovered in recent years.

The tax filing deadline in Canada is April 30, 15 days later than in the United States, but the CRA said it would give consideration to taxpayers unable to comply with their filing requirements because of the service interruption.

The CRA said later on Wednesday that it anticipated that online services would resume over the weekend.

(Reporting by Randall Palmer; Editing by Peter Galloway)